What is better to feed scratch or corn?

I feed Nutrena Nature Wise Layer 16% pellets. I am new to chickens. Do I need to feed scratch or corn? I feed them Nature Wise scratch in the yard or run. They get about 3 cups of scratch for 25 chickens. Is corn better than scratch? What is the diffrence in them? Also when is it best to feed that or is all year okay? They have free choice of layer,oyster shell, grit and clean water. Plus what ever they find when they get out for a few hours a day.

If you are feeding layer rations, neither scratch nor corn is necessary. They are just treats, and I believe either is fine as a treat. 3 cups of scratch or corn for 25 (I am assuming full sized chickens, not bantams) is not enough to cause any nutritional problems, as far as I know (and in my personal experience).

So I think you're fine either way. However, my own personal view is that more variation in the diet is better than less, so I give a mixture of scratch grains and bird seed (usually a mix of hulled sunflower, peanut hearts, and other small seeds such as niger and millet), and dried mealworms. My 15 birds (7 large and 8 bantams) get about 3 cups of that mix per day. And they are healthy and thriving.